- install Tor and use it for all subsequent browsing steps
- visit Instawallet to anonymously create a Bitcoin wallet
- List $100 cash for sale on Silk Road Market
- Mail the cash through any US Postal Service drop box
- When the Silk Road bitcoin payment arrives, withdraw it to your Instawallet
You can now spend Bitcoins from this Instawallet with complete anonymity. Instawallet doesn't know who you are (because of Tor). Silk Road doesn't know who you are (because of Tor). The USPS doesn't know who you are (anonymous drop boxes). The person who sold you Bitcoins doesn't know who you are (anonymous US mail).
Slightly more complicated: buy a Bitcoin mining rig and connect to a mining pool through Tor. Newly mined coins are completely anonymous. You can send them to your Instawallet or to a Bitcoin client running behind Tor.
There are hundreds of variations on these techniques, but the results are the same. High power forensics and digital surveillance might be able to pierce the anonymity veil. At that point, your attacker is probably motivated enough that Bitcoin anonymity is the least of your worries.